The study, which was conducted in Boston and Seattle by researchers from the Massachusetts Institute of Technology, Stanford University, and the University of Washington, found three notable trends: In Seattle, black people waited longer for Uber and Lyft drivers to accept their ride requests, and black riders waited up to 35% longer to be picked up by their UberX drivers. In Boston, Uber drivers cancelled on men with “African American sounding” names more than twice as often as on other men. Meanwhile, both ride-hailing companies took women in Boston for longer and more expensive rides than men.

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The paper comes at time when other so-called sharing economy companies are being called on the carpet over accusations of discrimination on their platforms. Airbnb, which has faced strong criticism from African American users, recently published an extensive report outlining its plan to reduce user bias.

Stephen Zoepf, co-author and executive director of the Center for Automotive Research at Stanford, says he suspects that some of the differences between the behavior of the Lyft and Uber drivers can be attributed to the variations in the two companies’ platforms. Uber does not show the driver the name of the fare until after booking, while Lyft provides drivers with the rider’s name and, in some cases, a photo before they accept the fare. “Lyft essentially makes it easier to discriminate upfront,” says Zoepf. Conducted over more than two years, the study included nearly 1,500 rides. In the Seattle test, a group of four black and four white research assistants (equally divided by gender) ordered cars over a six-week period. The Boston testers’ appearances “allowed them to plausibly travel as a passenger of either race,” though they requested rides under two different names, one “white sounding” and the other “distinctively black.”

Lyft spokesperson Adrian Durbin, emailed Fortune the following statement: “We are extremely proud of the positive impact Lyft has on communities of color. Because of Lyft, people living in underserved areas—which taxis have historically neglected—are now able to access convenient, affordable rides. And we provide this service while maintaining an inclusive and welcoming community, and do not tolerate any form of discrimination.”

While Uber did not respond to Fortune‘s request for comment, Rachel Holt, the company’s head of North American operations, told Bloomberg: “Ridesharing apps are changing a transportation status quo that has been unequal for generations, making it easier and more affordable for people to get around. Discrimination has no place in society and no place on Uber. We believe Uber is helping reduce transportation inequities across the board, but studies like this one are helpful in thinking about how we can do even more.”

While the variation in service for black riders consisted of longer wait times and more cancelled rides, the researchers found that, for women, the primary difference was how far they were driven on the typical ride—about 5% further than men.

The researchers note that some of the female test riders reported, “‘chatty’ drivers who drove extremely long routes, on some occasions even driving through the same intersection multiple times.” Their conclusion? “The additional travel that female riders are exposed to appears to be a combination of profiteering and flirting to a captive audience.”

The paper suggests a number of possible countermeasures to address the problems identified by the study, including avoiding the use of passengers names and imposing stricter penalties on drivers who cancel a ride. To address the issues related to female riders, the academics suggest upfront pricing, a service Uber is rolling out in New York City.

Zoepf notes that there are easy answers for ride-hailing companies, which must decide whether they want to focus on eliminating biased drivers or on implementing policies that ensure all their customers get the best service, regardless of drivers’ behavior. “There’s a real tension between trying to eliminate discrimination itself and trying to mitigate the effects of discrimination,” he says.